The invention relates to a magnetic actuator, and in particular to a magnetic actuator used with an iris diaphragm.
Optical devices such as optical projectors, rear projectors, or cameras comprise a light control device such as an iris diaphragm controller, adjusting light intensity to produce images of different light intensity. Conventional iris diaphragms are controlled by a dynamic or magnetic mechanism. When the conventional iris diaphragm controller is not in use, however, the light shield automatically returns to its default position, normally by spring force.
A conventional iris diaphragm controller comprising elastic restoring force is shown in FIGS. 1A to 1E, comprising a yoke 10, a first magnet 11, a second magnet 12, a coil 15, a light shield 16, a shaft 17, and a spring 18. The yoke 10 is a rectangular piece with a hole 107 defined therein. Conventionally, the yoke 10 comprises two U-shaped pieces with ends thereof facing each other. The first magnet 11 and second magnet 12 are disposed in the hole 107 of the yoke 10, respectively on opposing sides thereof. The first magnet 11 and second magnet 12 are arranged with opposite polarities facing each other, as shown in FIG. 1C. The North pole of the first magnet 11 is located at the left side of the coil 15, and the South pole thereof is located at the right side. The polarity of the second magnet 12 is opposite that of the first magnet 11 such that an upward magnetic field is generated on the right side of the coil 15, and a downward magnetic field is generated on the left side. The coil 15 comprises electric wires, located between the first magnet 11 and the second magnet 12. The shaft 17 extending from a side of the coil 15 comprises an opening 170. The light shield 16 is connected to the coil 15 on an opposite end of the shaft 17. The light shield 16 is normally disposed along the light path, perpendicular to thereto to block light. The shaft 17 has a spring 18 disposed thereon. The spring 18 has two ends thereof fixed on the shaft 17 and the yoke 10, respectively.
As shown in FIGS. 1C to 1E, when current passes through the coil 15 in the direction shown by the solid arrows in FIG. 1D, since current flows from right to left on the upper side of the coil 15, the magnetic field is perpendicular thereto and oriented from the FIG. 1D. According to the right hand rule, the coil 15 produces upward force. Since the shaft 17 is connected to a side of the coil 15, and another shaft (not shown) pivots on the opening 170, the coil 15 moves with respect to the opening 170 as a center point along the cross section line A-A′. That is, the coil 15 moves counterclockwise as shown by the hollow arrow of FIG. 1D. The lower side of the coil 15 is used as an example. When current flows from left to right, the magnetic field is perpendicular thereto and oriented into the FIG. 1D. According to the right-hand-rule, the coil 15 produces upward force. The coil 15 moves around the opening 170 as a center point along the cross section line A-A′, in a counterclockwise direction shown by the hollow arrow of FIG. 1D. As a result, the coil 15 moves the light shield 16 with respect to the opening 170, as shown in FIG. 1E in a counterclockwise direction. If current volume is adjusted accurately, angle of the circular motion of the coil 15 can be controlled such that the light shield 16 controls the size of the iris diaphragm.
If there is no current through the coil 15, magnetic field between the coil 15 and the first and second magnets 11, 12 is not induced. After the spring 18 is extended, an elastic restoring force is generated. Consequently, the coil 15 is moved with respect to the opening 170 along the cross section line A-A′ in a clockwise direction, as shown by the hollow arrow in FIG. 1E. Thus, the spring 18 returns the coil 15, the shaft 17, and the light shield 16 to their original position.
However, the structure of the coil is complicated and difficult to fabricate, elevating manufacturing costs. Furthermore, since the magnetic fields generated by the magnetic structure thereof are not uniform, the relationship between the current volume in the coil and the circular motion is difficult to control. To achieve sufficient actuating force for the magnetic actuator, volume is increased. The width thereof cannot be reduced. Moreover, restoring force produced by the spring increases with extension of the spring, making it difficult to control. Lifetime of the spring is short, with elasticity thereof depleting over time.